This invention relates to a process for producing resin impregnated fiber reinforced materials and is particularly concerned with novel procedure for impregnating dry fiber reinforcement, particularly carbon or graphite cloth, with a predetermined amount of resin, so as to impregnate such fiber reinforcement rapidly and uniformly throughout the fiber reinforcement, followed by final curing, to produce a resin impregnated reinforcement material which is essentially completely and uniformly impregnated with resin.
The concept of impregnating dry fibrous materials, such as glass fiber, with a resin under vacuum conditions and curing the resin to produce a composite is known in the art, as disclosed, for example, in U.S. Pat. Nos. 3,028,284; 3,322,566; 3,384,505; 3,523,152; 3,790,432; and 4,030,953.
Fiber reinforced organic resin composite structures are fabricated using two basic forms of materials, namely, prepreg ("B" stage) resin impregnated fiber forms and wet resin impregnation of fiber forms.
In the prepreg process, woven cloth or yarn or fiber tow is impregnated at one facility, with a prescribed amount of organic resin. The resin is staged and dried, usually to a "tacky" "B" stage condition, that is, in a partially cured condition. The material is then packaged between layers of separation film and stored in sealed containers, usually at low temperature, for extended periods of time before it is used and fully cured for final part processing.
The prepreg operation has a number of disadvantages. It requires a separate manufacturing facility, and the "B" stage material usually must be stored at low temperature and in sealed bags to avoid contact with moisture. The resins must be "conditioned" to a specific state of polymerization or tack, and then the process must be stopped to retain this tack condition over an extended period of time, and thereafter, the resins must still have the proper processing characteristics for final curing. Further, the prepreg material is generally laid up by hand, and there is much direct personal contact with the "B" stage resin, which is undesirable.
In the wet resin impregnation process, woven cloth or yarn is impregnated with a liquid resin that is catalyzed to process or cure in a short continuous period of time. In this process, the resin is usually impregnated by squeegee of ply by ply of a layup at the site of component fabrication. The impregnated material can be handled at room temperature or elevated temperature for a certain time period during which the resin gels, followed by final curing either at room temperature or elevated temperature in the same tool or mold.
Disadvantages of the wet resin process are that it is messy and there is much direct personal contact with the resin, which is undesirable, and it is difficult to obtain the desired amount and uniform resin content free of voids and bubbles. Wet resin content fabricated products are usually of higher resin content than similar "B" stage fabricated products, in order to assure freedom of void within the laminate, and thus, such panels are heavier than panels made with "B" stage materials.
In one form of the vacuum impregnation wet resin process, resin and catalyst systems are mixed in a container, degassed, and then introduced from the container directly to a dry cloth fiber reinforcement layer or layup, placed on a tool. A vacuum bag is placed over the dry cloth layup with an inlet tube from the resin container to an edge of the layup under the vacuum bag. The vacuum bag outlet to the vacuum source is at the center of the assembly. When a vacuum is pulled, the bag pulls against the layup, and when the resin is released, it passes through the tube from the resin container and impregnates the dry fiber reinforcement or cloth from the edge thereof, and thereafter, resin flow proceeds toward the vacuum outlet at the center of the fiber reinforcement. When the resin reaches the vacuum outlet, the part is impregnated, and the resin inlet is sealed to stop any additional resin flow. The cure cycle is completed with continued vacuum pressure and heat.
While this process, as described above, works satisfactorily for fabrication with medium-size fiberglass reinforcement panels, e.g., of about 3 feet by 4 feet, when impregnated with, for example, epoxy resin, the process does not satisfactorily impregnate close weave fiber reinforcement, such as carbon fiber panels, entirely along the length and width thereof, to useful large size, using the same resin system and technique. The fiber wetting characteristics are substantially very different between fiberglass and carbon cloth reinforcements. Using presently practiced wet resin processes, the obtainment of close thickness or tight resin content tolerance throughout the impregnated fiber reinforcement, from the resin inlet area to the vacuum outlet side of the fiber reinforcement, is difficult to achieve.
U.S. Pat. No. 4,311,661 to Palmer discloses a resin impregnation process employing either a "B" stage resin or a wet resin, such as a liquid epoxy resin. However, in this arrangement, whether a "B" stage resin or a liquid resin is employed, the resin material is in the form of a layer on the tool and is entirely enclosed within the vacuum bag as in the form of a cocoon. Thus, in this patent, for example, a "B" stage resin film or a wet resin film is applied to a mold surface, a fiber reinforcement layer, such as fiberglass, is placed thereover, a bleeder layer is placed over the fiber reinforcement layer, and a non-porous material is positioned over the bleeder layer. A vacuum bag is placed over the arrangement, a vacuum is applied to the mold, and the mold is heated to a temperature sufficient to cause the "B" stage resin to flow and to impregnate through the thickness of the reinforcement layer and to impregnate the bleeder layer. The mold pressure is then increased, and the temperature is raised to produce final curing of the resin impregnated fiber reinforcement layer.
It is an object of the present invention to provide an improved wet resin impregnation process for impregnating dry fiber reinforcement material, particularly carbon fiber reinforcement, so as to produce uniform impregnation of resin into the fiber reinforcement material from side to side and from one end to the other or, if desired, through the thickness of the reinforcement material.
Another object is the provision of a process for wet resin impregnation of a fiber reinforcement so that it is uniformly saturated or impregnated with a predetermined desired resin content from side to side and from end to end of the fiber reinforcement layer.
Another object is to provide a wet resin process for uniformly impregnating fiber reinforcement material with a thermosetting resin, and curing the resin, directly on a tool or mold.
A further object is to provide a process for rapidly impregnating fiber reinforcement material which can be scaled up to large-size components without need for expensive autoclave equipment.
Another object is the provision of a process for producing sandwich panels directly on a tool by uniform resin impregnation of fiber reinforcement skins positioned on the outer surfaces of a supporting member, such as a foam core.
A still further object is the provision of a process for resin impregnation of a fiber reinforcement which can be carried out in a closed system, and wherein there is no physical contact by personnel with the resin and no escape of volatiles from the closed system to the exterior ambient atmosphere.
Yet another object is the provision of systems for carrying out the above processes.